In all radio receivers, the first amplifier on the path from the antenna into the receiver should have very low noise characteristics, for the signal level at the input of this amplifier is low and additional noise caused by the amplifier will be amplified in all amplifier stages to follow. A low-noise first amplifier is usually abbreviated LNA. In receivers of mobile network base stations the maximum allowable noise figure specified for the LNA is typically 1.8 dB. This is a relatively strict requirement especially considering that the figure includes the increase in the noise figure caused by a circuit arrangement enabling bypassing of the LNA. The LNA needs to be bypassed because of certain base station maintenance measurements and in LNA fault conditions.
FIG. 1 is a block diagram illustrating the front end of a typical RF receiver, and a known arrangement of bypassing the LNA. FIG. 1 shows an antenna ANT and from there on, on the receiver's signal path, an antenna filter 110, a first switch SW1, low-noise amplifier LNA, second switch SW2, bandpass filter BPF, and a mixer MIX. The mixer gives an intermediate-frequency signal for further processing. Switches SW1 and SW2 are changeover switches having a common control C. When the switches are in position 1, the output signal from the antenna filter 110 is guided via switch SW1 to the LNA input, and the LNA output signal via switch SW2 towards the intermediate-frequency part. With the switches in position 2, the output signal from the antenna filter 110 is guided via switch SW1 to switch SW2 and from there on towards the intermediate-frequency part. The LNA is thus bypassed. In this patent application the receiver part 120 comprised of the LNA and its bypass arrangement is called a front stage.
The first and second switches SW1, SW2 may be implemented by PIN diodes, MMIC (Microwave Monolithic Integrated Circuit) components, MEMS (Micro-Electro-Mechanical System) switches or relays, for example. Attenuation in a signal caused by a single switch is on the order of 0.25 dB. Attenuation in the switch in front of the LNA increases the noise figure of the front stage in the same proportion; at the output side of the LNA the influence of the switch on the noise figure is smaller. Another disadvantage caused by the switches is that in the normal operating mode of the receiver the isolation on the LNA bypass path is finite, not infinite. This, too, degrades the front stage noise figure. Switches can be designed so as to have a very high isolation, but then, in practice, they induce greater losses.